Dark features in the solar corona and the underlying magnetic field studied with Solar Orbiter

The solar corona shows a variety of structures characterised by significantly reduced emission in the extreme ultraviolet (EUV). These include coronal holes (CHs), coronal dimmings, filaments, dark halos around active regions (ARs), and smaller-scale features such as coronal voids. While they share the common property of diminished EUV brightness, the physical conditions that give rise to these structures can differ greatly, and are, in some cases, not yet fully understood. This thesis combines two complementary studies of EUV-dark structures, focusing on a very first description of features we termed coronal voids embedded in the quiet Sun (QS) and on dark halos surrounding ARs, both aimed at exploring the relationship between EUV emission and the underlying photospheric magnetic field in these low-emission structures. For these studies, we utilised the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter to capture high-resolution observations of the solar corona, providing detailed imaging of fine-structures. These observations were complemented by co-temporal line-of-sight magnetograms from the Polarimetric and Helioseismic Imager (SO/PHI), which allow for a direct comparison between the coronal emission and the underlying photospheric magnetic field. The study of dark halos surrounding ARs additionally uses supplementary data from the Solar Dynamics Observatory (SDO). The first study examines dark coronal features in the QS that had not been identified or investigated before. These typically span scales from a few granules to several supergranules (∼30 Mm). These coronal voids were identified in EUI data using a 75% intensity threshold of the mean QS emission and exhibit an average brightness of only 67% of the surrounding field of view. To investigate the origin of their reduced emission, two competing hypotheses were considered. One proposes that coronal voids are miniature analogues of CHs, with one dominant magnetic polarity and an open magnetic field configuration, which would account for a net loss of plasma and lower emission. The second hypothesis suggests that these regions appear dark because of reduced coronal heating resulting from a locally weak photospheric magnetic field. To test both scenarios, a co-temporal SO/PHI magnetogram was analysed to assess the underlying magnetic flux. The results show a significant reduction in magnetic flux density by at least 76%—compared to adjacent QS areas, and an absence of strong magnetic network structures. These findings support the second hypothesis, indicating that the dark appearance of coronal voids is primarily a consequence of reduced magnetic heating of the corona. The results identify coronal voids as a previously unrecognised category of EUV-dark structures, distinct from CHs in both scale and magnetic configuration. The second project addresses a different class of EUV-dark regions: coronal dark halos surrounding ARs. These structures show significantly reduced emission at temperatures at or below 1 MK. Although they are sometimes mistaken for CHs, they differ substantially in their magnetic and thermal characteristics. The mechanisms underlying their formation remain unclear, despite several proposed models. This study takes a new approach by analysing the variation of both coronal emission and photospheric magnetic field strength as a function of radial distance from the centre of AR NOAA 12893. High-resolution observations from Solar Orbiter’s EUI and SO/PHI instruments were combined with SDO data to probe the coronal plasma across a broader temperature range. The results show that while emission at lower coronal temperatures remains uniformly suppressed across the dark halo, the hotter component above 1.6 MK reveals a clear radial gradient: emission levels decrease with distance from the AR, eventually approaching QS values. Similarly, the underlying magnetic field is generally weaker within the dark halo compared to adjacent bright areas. Compared with the QS, however, the field in the dark halo is stronger close to the AR, but decreases towards its outer boundary, falling below the QS mean on the mean. Near its outer edge, it takes on values typical of those found in coronal voids. These findings point to the dark halo’s reduced emission, especially in the outer regions, arising from the diminishing magnetic field that likely limits local heating. In the areas of the dark halo with stronger magnetic fields closer to the AR other mechanisms are anticipated to act in addition to the reduced heating. This thesis offers new insights into the magnetic and radiative properties of EUV-dark structures in the solar corona, utilising snapshot observations. In future, expanding this research to include time-series observations would allow for a detailed analysis of the evolution and dynamics of these structures. Initial considerations for such future investigations are also discussed, paving the way for a more comprehensive understanding and continued exploration.